Tardigrades are microscopic animals and they are awesome. They can withstand more abuse than any other known animal. They can tolerate heat, cold, dehydration, and even the vacuum of space! But they are not extremophiles (critters that love environments that other critters would die in). They are extremotolerant. This means that tardigrades don’t like to be boiled or frozen or having all their fluids sucked out by vacuums, they just put up with it and go about their business.
However, they’re not just tough, these little critters are also funny looking, ancient, and they have extremely healthy sex lives!
First a disclaimer: there are over 1,000 species of tardigrades and some are much more robust and interesting than others. However, in this post I simply refer to “tardigrades” and not particular species.
Tardigrade physical description:
Imagine a microscopic sumo-hippo-spider-bear with claws and a fast-action extendable, tubular mouth. Tardigrades are sort of like that (Figure 1).
Tardigrades only grow to about 0.5 mm (0.02 inches) in length. Thus, they typically can’t be seen without a microscope. Overall, they are sort of jolly, fat, Stay-puffed Marshmallow-Manish in appearance (Figure 1) with a tan or slightly transparent complexion. They have eight legs that all end in tiny hooked claws. And, often (depending on the particular species) they have a cylindrical snout that telescopes outward to catch food (GIF 1). Sort of like Aliens do when they’re sniffing Sigourney Weaver or the “Pinchers of Peril” mouths of a goblin shark.
Tardigrades are found pretty much everywhere. They live where it’s deep (below 4,000 m or 13,000 ft in the oceans). They live where its high (over 6,000 m or 20,000 ft in the Himalayas). And they span the Earth’s tropics and poles.
They inhabit all kinds of micro-environments from lichens to beach sand. Typically, they can be found in mosses—thus the name “moss piglets”. Fossil evidence shows that they have been living like this since the Cambrian Period, 530 million years ago. That means that they have survived all five of the known mass extinctions!
Tardigrade sex life:
Tardigrades seem to have quite a healthy sex life. They mate in a sort of cuddle-coitus position where the male curls around the female’s face (Lemloh et al., 2011). All of the copulation activities take place at the head-end of a tardigrade. None of those waste-processing-plant-near-recreation-area type problems for them. And, apparently they enjoy sex quite a bit. They copulate for about an hour while the male ejaculates several times.
Probably the primary reason for why tardigrades should be everyone’s favorite animal is their toughness (Figure 2). The following four sections will outline some of the down-right drastic things tardigrades can handle and how they do it.
They tolerate ridiculously cold temperatures, down to −458°F (−272°C)—which is very close to absolute zero (1 K)! And they can handle being cold for a long time. One species of Antarctic tardigrade has even survived for 600 days at a constant temperature of -7.6°F (-22°C) (Sømme and Meier, 1995). But, unlike other cold-tolerating animals, they can also endure temperatures of up to 300 °F (150 °C)!
They can withstand extremely high pressures. Tardigrades that live in the Earth’s deepest ocean trench (the Marianas Trench) cope with pressures of about 100 MPa. How much is that? Well, at sea level, we all experience about 0.1 MPa of atmospheric pressure. At around 300 ft underwater free divers often lose consciousness and die. At this depth, divers are experiencing only about 1.0 MPa. But even the deepest ocean trench is just peanuts to a tardigrade. In laboratory settings, tardigrades and their eggs have survived prolonged exposure to 7.5 GPa of pressure (Minami et al., 2010). That’s 7,500 MPa—enough to crush us wimpy humans to death in an excruciating show of farts and oozes!
Here is some context on how much 7,500 MPa really is: at these pressures DNA is ripped apart and the fat that forms cell membranes solidifies from a gel (which is good) into a “liquid crystalline state” (Bartlett, 2002). Even bacteria die at 300 MPa.
However, Tardigrades’ real claim to fame comes from the fact that they can withstand extremely low pressures. They can even survive in the vacuum and cosmic radiation of space (Jönsson et al., 2008; Persson et al., 2011). Obviously exposure to space would kill humans, or just about anything else right away. But Tardigrades have survived exposure for twelve continuous days in space (Rebecchi et al., 2009).
It’s true that the vacuum of space is hard to contend with. Humans would not last long at all. The lack of pressure would form bubbles in all of your body’s precious fluids within seconds. You wouldn’t boil or burst, but you’d swell up pretty bad and be in a lot of pain. Also, if you held any air in your lungs it would expand and pop your lungs like cheap balloons. However, surviving the vacuum isn’t the only hurdle for excessive exposure to space. Tardigrades also withstand a lot of cosmic radiation. Researchers have exposed tardigrades to very large doses of gamma rays and heavy ions. Results show that they can survive many times more cosmic radiation than any other know animal; although at large doses they are rendered sterile (Horikawa et al., 2006). So, what would kill us humans is just a drastic form of birth control to tardigrades!
How do Tardigrades manage to be so resilient?
A big part of tardigrade resilience to environmental stress is due to their ability to desiccate. This adaptive strategy is called anhydrobiosis. When a tardigrade undergoes anhydrobiosis it loses almost all of its water content. A dehydrated tardigrade is said to be in its “tun” state. When a tardigrade is in tun, its metabolism almost stops completely. So it is like they are in a super hibernation.
So how do they survive this severe dehydration? Well they do this by producing a lot of trehalose. Trehalose is a sugar that takes on a protective glass-like form inside tardigrade cells. This sugar also helps stop any remaining water from expanding if it’s frozen or heated, which could rupture the cell. So, during their super-hibernation state, tardigrades are also physically tougher than usual.
However, anhydrobiosis isn’t the only tool in a tardigrade shed. They also have good genes. Really good—not those skinny-legged things the kids are all wearing these days. One recent genomic study shows that tardigrades have four copies of a gene that code for DNA-repairing proteins (Hashimoto et al., 2016). This means that tardigrades are really great at repairing physical damage.
But that’s not all! One of the tardigrade genes actually helps to prevent physical damage at the DNA scale! This protein is called Dsup (short for damage suppressor). Recent studies actually made a direct observation of Dsup actively migrating through tardigrade cells (Hashimoto et al., 2016). They found that the Dsup actually wraps itself around DNA to protect it from radiation damage.
Discussion: what we get from tardigrade research:
Why study tardigrades? Well first we have to get the obvious answer out of the way. This research is interesting and it inspires future generations to take an interest in the world around them.
Less obvious is the fact that research into tardigrades (like all good research) sharpens our knowledge of the world around us. And these studies can help to refine certain methods through the exposure of disagreements and
corrections. For example, studies examining the amounts of tardigrade foreign DNA produce results from 17% (Boothby et al., 2015) to ≤1.2% (Hashimoto et al., 2016). (By the way, we now know that the 17% result is likely due to contamination.)
Tardigrade research may even prove useful to the preservation of the human species! Scientists found that by inserting the gene that codes for the Dsup protein into human kidney cells the cells experienced 40% less DNA damage from radiation exposure (Hashimoto et al., 2016). This could conceivably prove quite useful for when us humans start exploring the solar system and all the cosmic radiation that comes with it. Because let’s just face it, eventually—and for whatever reason (killer asteroids, killer climate, killer wars, or just plain-old drive for adventure)—we will need to leave this planet and check out the rest of the galaxy.
Tardigrades are the best. They are interesting and we can learn a lot by studying them.
Also, since they are nearly indestructible, they likely don’t worry about much. Which is the inspiriation for my “Partigrade” graphic (Figure 3).
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Boothby, T. C., Tenlen, J. R., Smith, F. W., Wang, J. R., Patanella, K. A., Nishimura, E. O., … & Messina, D. N. (2015). Evidence for extensive horizontal gene transfer from the draft genome of a tardigrade. Proceedings of the National Academy of Sciences, 112(52), 15976-15981. http://www.pnas.org/content/112/52/15976
Hashimoto, T., Horikawa, D. D., Saito, Y., Kuwahara, H., Kozuka-Hata, H., Shin, T., … & Enomoto, A. (2016). Extremotolerant tardigrade genome and improved radiotolerance of human cultured cells by tardigrade-unique protein. Nature Communications, 7. http://www.nature.com/articles/ncomms12808
Horikawa, D. D., Sakashita, T., Katagiri, C., Watanabe, M., Kikawada, T., Nakahara, Y., … & Kobayashi, Y. (2006). Radiation tolerance in the tardigrade Milnesium tardigradum. International journal of radiation biology, 82(12), 843-848. http://www.tandfonline.com/doi/abs/10.1080/09553000600972956
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